1. Field of the Invention
The present invention relates to a light-emitting device using an organic compound and a display apparatus using the device, and more specifically, to an organic light-emitting device (hereinafter, also simply referred to as device) that emits light when an electric field is applied to a thin film made of an organic compound, and a display apparatus using the device.
2. Description of the Related Art
Research and developmental works are now carried out actively on organic light-emitting devices (organic EL devices or organic electroluminescent devices). In such organic EL devices, typically, moisture or oxygen in an external environment causes deterioration of an organic compound, deterioration and peeling of an electrode/organic-substance interface, oxidization of electrodes, and the like. This results in a reduction of emission luminance, an increase of a driving voltage, generation and growth of a non-emission portion called a dark spot, and the like. As a result, there has been a problem concerning the reliability of the display apparatus.
In order to solve this problem, Japanese Patent Application Laid-Open No. 2001-357973 discloses a display apparatus in which a sealing layer constructed by stacking a plurality of inorganic material films having different moisture absorption characteristics is bonded to a light extraction side of a top emission type device.
FIG. 9 illustrates an example of the display apparatus disclosed in Japanese Patent Application Laid-Open No. 2001-357973. FIG. 9 illustrates a substrate 1, an organic light-emitting device 2, a silicon nitride film 3, a silicon oxide film 4, a silicon nitride film 5, a resin 6 that bonds a sealing layer and a glass substrate together, and a glass substrate 7. The sealing layer is constituted by three layers of the silicon nitride film 3, the silicon oxide film 4, and the silicon nitride film 5.
As for the film thickness of the sealing layer disclosed in Japanese Patent Application Laid-Open No. 2001-357973, the silicon nitride film 3 has a thickness of 2 to 3 μm, while the silicon oxide film 4 and the silicon nitride film 5, respectively, have a thickness of about 1 μm. Thus, the total thickness of the sealing layer is approximately 4 to 5 μm. Further, for the glass substrate 7, since processing into a cap shape is unnecessary, a comparatively thin glass substrate may be selected. Thus, a combination of a sealing layer obtained by stacking a plurality of layers of inorganic materials and a thin plate glass substrate may be used so that a thinner display apparatus can be realized in comparison with an apparatus obtained by a conventional technique using a sealing cover.
For the above described structure where a sealing layer is formed on the light extraction side of a top emission type device so that moisture or oxygen is prevented from entering, an organic light-emitting device is disclosed, which has a microcavity (or micro-resonator) structure for the purpose of improving the light extraction efficiency or achieving the emission of light having a desired chromaticity.
In an organic light-emitting device disclosed in Japanese Patent Application Laid-Open No. 2003-109775, a light extraction electrode (a second electrode) is constituted of a translucent reflective layer and a transparent conductive layer. In the organic light-emitting device, on the above-mentioned electrode, a passivation film having a refractive index comparable to that of the material constituting the electrode is formed in a thickness of 500 to 10,000 nm so that the device surface is protected. In this organic light-emitting device, the translucent reflective layer constituting a part of the light extraction electrode is formed of a thin film of silver or an alloy containing silver as a main component. In this device, a microcavity structure is formed between a reflective first electrode formed on a substrate side and the translucent reflective layer constituting a part of the light extraction electrode, and an organic compound layer interposed between the first electrode and the translucent reflective layer serves as a resonance portion.
In this organic light-emitting device, the optical distance between the first electrode and the translucent reflective layer, that is, the optical film thickness of the resonance portion is represented by L, and the phase shift generated when a light emitted from an emission layer is reflected by the first electrode and the translucent reflective layer is represented by φ (radian). Further, when the resonance wavelength of the microcavity structure is represented by λ. Then, the following Equation 1 is satisfied.λ=1/2Lx(m−φ/2π) (m is an integer)  (Equation 1)
It can be seen from Equation 1 that the resonance wavelength λ varies depending on the optical film thickness L of the resonance portion, and hence the resonance effect of the microcavity structure can be adjusted. Incidentally, the optical film thickness L is a total (n1d1+n2d2+ . . . ) of the optical film thicknesses (i.e., refractive index (n) ×film thickness (d)) of the respective organic compound layers interposed between the first electrode and the translucent reflective layer.
In an organic light-emitting device having such a microcavity structure, a light exiting to the outside of the device receives the effect by the resonator as expressed by Equation 1, so that the emission characteristics thereof such as emission luminance and chromaticity are changed. That is, by adjusting the resonance effect, the emission characteristics of the device can be controlled.
FIG. 10 is a schematic cross-sectional view of an organic light-emitting device having a microcavity structure. FIG. 10 illustrates a substrate 1, a reflective electrode 8 serving as an anode, a transparent conductive layer 9, a hole-transporting layer 10, an emission layer 11, an electron-transporting layer 12, and an electron injection layer 13. Further, FIG. 10 illustrates a translucent reflective layer 14 made of silver, a transparent electrode 15 made of IZO, a silicon nitride film (sealing layer) 4, and light E emitted from the organic light-emitting device. Here, the silicon nitride film 4 formed on the transparent electrode 15 corresponds to the passivation film in Japanese Patent Application Laid-Open No. 2003-109775. In this organic light-emitting device, a microcavity structure is formed between the reflective electrode 8 and the translucent reflective layer 14, and the light E passes through the sealing layer 4 from the transparent electrode 15 side to the outside.
Here, using a general-purpose calculation software, a simulation analysis was carried out for the emission colors of an organic light-emitting device having the sealing layer disclosed in Japanese Patent Application Laid-Open No. 2003-109775 and shown in FIG. 10 and an organic light-emitting device having neither a sealing layer nor a translucent reflective layer as shown in FIG. 11. FIG. 10 is a schematic cross-sectional view of the organic light-emitting device disclosed in Japanese Patent Application Laid-Open No. 2003-109775. FIG. 11 is a schematic cross-sectional view of an organic light-emitting device having none of a sealing layer and a translucent reflective layer.
The film thicknesses of the hole-transporting layer and the electron injection layer of each of such organic light-emitting devices that have a microcavity structure and emit blue light were variously changed, and the emission characteristics were estimated by the simulation analysis. Tables 1 and 2 show the film thicknesses of the layers and the chromaticity changes of the emission color of the analyzed organic light-emitting devices.
TABLE 1Film thickness configurationFIG. 10FIG. 11Silicon nitride layer2,000 nm—Transparent conductive film60 nmTranslucent reflective layer  20 nm—Electron injection layer10-100 nm, 5 nm stepElectron-transporting layer10 nmEmission layer20 nmHole-transporting layer 10-70 nm, 5 nm stepTransparent conductive layer10 nmReflective electrode100 nm Substrate635,000 nm   
TABLE 2Emission chromaticity change by film thicknessadjustmentChromaticityadjustment rangeFIG. 10FIG. 11CIExLower limit0.1210.131Upper limit0.4020.327Adjustment range0.2810.197CIEyLower limit0.0660.077Upper limit0.5260.534Adjustment range0.4600.458
It can be seen from Table 2 that in the organic light-emitting device having a microcavity structure, when the resonance effect is adjusted by means of the film thickness, light emission of approximately 0.066 in terms of the chromaticity coordinate CIEy value can be achieved. That is, a blue color which is deeper than that in the case of the organic light-emitting device shown in FIG. 11 can be reproduced.
As described above, in the organic light-emitting device having a microcavity structure shown in Japanese Patent Application Laid-Open No. 2003-109775, even when a member that exerts an optical influence, such as a sealing layer, is formed, the emission color can be adjusted. Further, the degree of freedom in the adjustment is higher than that of a conventional organic light-emitting device having no sealing layer.
Such adjustment of the emission characteristics is achieved by the strong resonance effect of the microcavity structure. Incidentally, also in the organic light-emitting device with the structure shown in FIG. 11, the refractive index difference between the transparent electrode 15 and the external environment (dry air) causes a part of emission light to be reflected by the interface. The light reflected by the interface resonates between the interface and the reflective electrode on the substrate side. That is, the organic light-emitting device shown in FIG. 11 also has a microcavity structure.
Nevertheless, a difference is present in the resonance effect between the organic light-emitting device shown in FIG. 11 and the device disclosed in Japanese Patent Application Laid-Open No. 2003-109775 and shown in FIG. 10. This results from a difference in the reflectance of the light extraction side reflective portion.
FIG. 12 shows the results of an estimation using a general-purpose calculation software carried out for the reflectance at the interface between the transparent electrode 15 (IZO) of the organic light-emitting device of FIG. 11 and air, and the reflectance at the interface of the electron injection layer 13—translucent reflective layer 14 (20 nm)—transparent electrode 15 of the organic light-emitting device of FIG. 10.
As can be seen from FIG. 12, the reflectance of the device described in Japanese Patent Application Laid-Open No. 2003-109775 in which the translucent reflective layer is used, is higher than the reflectance at the interface between the transparent electrode 15 and the air. This causes a difference in the resonance effect between these microcavity structures.
In the organic light-emitting device of Japanese Patent Application Laid-Open No. 2003-109775, the reflectance of the translucent reflective layer on the light extraction side is sufficiently high. Therefore, even when the sealing layer is provided, the emission characteristics can be adjusted by using the strong resonance effect. Thereby, an organic light-emitting device can be realized that has a smaller thickness and whose emission characteristics can be controlled arbitrarily.
Nevertheless, in the organic light-emitting device having a microcavity structure disclosed in Japanese Patent Application Laid-Open No. 2003-109775 above, the resonance effect is determined by the film thickness and the refractive index of the organic compound layer. Therefore, a change in these parameters affects the emission characteristics of the device. Accordingly, if there is nonuniformity in a film thickness within a substrate surface where the organic light-emitting device is formed, or if there is a variation in film thickness between a plurality of organic light-emitting devices, the emission characteristics varies depending on the film thickness difference, which has posed a problem that it is difficult to produce devices having the same emission characteristics repeatedly with satisfactory reproducibility.
Tables 3 and 4 below show a change in the emission chromaticity in a case where the film thicknesses of the respective layers including the organic compound layer and the sealing layer that constitute the ordinary organic light-emitting device shown in FIG. 11 and the organic light-emitting device having the microcavity structure shown in FIG. 10 are changed uniformly within the range from −10% to +10%. The film thicknesses of the organic light-emitting devices at this time are as shown in Table 1. Incidentally, the term “change range” employed in the tables refers to a difference between an upper limit value and a lower limit value of each chromaticity coordinate.
TABLE 3(Organic light-emitting device)Change in emission chromaticity due to film thicknesschange−10%−5%0%+5%+10%Change rangeCIEx0.1530.1490.1440.1370.1330.020CIEy0.0860.0890.1060.1420.1970.111
TABLE 4(Microcavity type device)Change in emission chromaticity due to film thicknesschange−10%−5%0%+5%+10%Change rangeCIEx0.1450.1390.1310.1200.1110.033CIEy0.0860.1040.1340.1840.2620.175
As can be seen from Tables 3 and 4 above, the emission chromaticity of the device (microcavity type device) having a microcavity structure varies more greatly than the organic light-emitting device to which similar film thickness changes are imparted. That is, the emission characteristics of the device having a microcavity structure are sensitive to a film thickness change of each layer. Therefore, for the purpose of stabilization of the emission characteristics, there are required a film formation process having the controllability of film thickness improved in comparison with an ordinary process and a control system and a process step having higher precision for the film thickness. These requirements have caused the problem of a low throughput of the device production in the case of mass production of the microcavity type device.